You won't necessarily find a term that's equivalent to southpaw that's as common or on the same register. That's because being right-handed is considered the default, unmarked condition, and as such doesn't need to be explicitly named. Left-handedness, being the marked condition, will usually develop a specific word.
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Avner Shahar-KashtanAug 3 '12 at 8:40

Furthermore, the French word for right-handed is droitier and maladroit ("inept; clumsy; awkward; not adroit"), while rarely used, can also mean left-handed. The more mainstream term for left-handedness in French is not all that much better, as it is gauche ("Awkward or lacking in social graces; bumbling.").

Interesting. I've always thought lefties tend to be slightly more ambidextrous than righties (simply because they are steered toward using their right hands – far more so than right-handed people using their left hands). After thinking about that some more, though, even if that premise is true, it's likely to be a modern phenomenon.
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J.R.Aug 3 '12 at 10:10

@J.R. It might then follow that, in pre-modern times when lefties were forced to become righties in many cultures, they would have become more ambisinistrous than if otherwise :)
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coleopteristAug 3 '12 at 13:29

@J.R.: I believe that genetic studies have shown that there's a right-handed gene. People without it can be either left-handed or right-handed or ambidextrous; they're slightly more likely to be right-handed, but this is probably cultural. So I believe you're correct: lefties (and righties without this gene) are more likely to be ambidextrous.
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Peter Shor Aug 3 '12 at 15:30

@PeterShor: That's an interesting take, but I was referring more to practice (through 'forced' use) than genetics. Several modern contraptions are designed for right-handed people, although that may escape notice if one is not paying attention. Scissors are perhaps the most obvious example, but consider credit card swipe readers: if the slot is not on the top, it's usually on the right. Even the mouse I'm using now is designed to have the primary click be the right-hand index finger. Southpaws are simply forced to use their right hands more than righties are their left, so, more ambidexterity.
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J.R.Aug 3 '12 at 21:22

In boxing, the two terms are used to represent the stance rather than the handedness.
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coleopteristAug 3 '12 at 6:22

@coleopterist: Yet the stance has everything to do with handedness. For boxers who take an orthodox stance, the left foot is forward and the jabs are made with the left hand, while the power punches — the ones involving a full swiveling extension of the hips and shoulders — are delivered by the right hand. This is reversed for the southpaw stance, in which the right foot is forward and knockout punches are delivered with the left hand.
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RobustoOct 20 '12 at 14:48

@Robusto Some boxers who are left-handed (like Miguel Cotto) fight orthodox. Other boxers (such as Prince Naseem) regularly switch stances. The change in stance gives them a far stronger jab as it is executed using their dominant hand. While usually true, a southpaw boxer is not always left-handed and an orthodox boxer is not always right-handed.
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coleopteristOct 20 '12 at 15:34

Coleopterists answer is true for a range of languages including gaelic. The right hand is associated with skill and grace and the left with clumsiness. CF "cack-handed". Some of the online dictionaries misattribute the "cack" (excrement) as "because clumsy people make a mess". The left hand was traditionally the hand you wiped your bottom with and the right the hand with which you ate (see various Roman and Islamic references for starters). When sharing food you wanted to be sure the person used different hands for cleaning himself and touching food.